The production of paper involves many process variables which can affect paper quality and formation consistency. For example, the consistency of wood fibers and lignin content in the pulp can vary as wood from different types of trees is introduced into the stock. If, for example, lignin content increases or short fibers content decreases, machine settings must be commensurately adjusted and controlled to maintain consistency in the end product. Wood composition can vary even among trees of the same type growing in the same geographic location based on varying environmental factors such as soil composition, amount of sunlight, rain, etc.
As a further production variable, formation characteristics of paper are, to a large extent, unique to a particular papermaking machine. Papermaking machines are typically large with no two machines being built to the same specifications, dimensions, and operating speeds. Different machines are also operated and maintained differently. The result of these and other process variations in papermaking machines is that paper formation characteristics will vary in accordance with the particular machine used during paper production.
The many process variables involved in the manufacture of paper must be adequately and consistently controlled to avoid undesirable paper formation properties. Visual characteristics of the paper (including formation, moisture streaks, wire marks, dirt, roughness, coating uniformity, gloss variation, and misregister in printing) can provide much needed feedback for machine operators to adjust process variables in order to achieve a consistently high quality paper product. Unfortunately, current techniques fail to adequately monitor these visual characteristics in a manner that would enable machine operators to make needed production adjustments in an effective and efficient manner.
Control of process variables during production is also important because many processes today utilize recycled pulp, which varies significantly in the quantity of undesirables and in the consistency of fiber characteristics. Recycled pulp contains varying amounts of undesirables such as printing ink, polymers from coatings, dirt, and other contaminants. Also, fiber and lignin content vary significantly because recycled pulp is typically made from a variety of paper products that were also produced by different processes with different machines and from different wood sources. Therefore, production control can become even more critical to formation consistency when using recycled pulp.
A principle factor that has frustrated the evolution of a new generation of pulp control systems is the inability to accurately, reliably, and consistently observe visual characteristics of the paper product as it is formed. A common production control technique is to utilize human observation of the paper's visual characteristics. By holding a sample paper web up to a light source, one is able to observe, in general measure, formation consistency and other visual characteristics within the paper web. This type of direct human observation of the paper's visual characteristics provides a good general assessment of formation, but it is inconsistent and inefficient. Human observation is largely subjective and prior attempts to provide a system for valuating human observed characteristics are ineffective. Human assessment is ineffective because the human eye is unable to observe the paper web on-line as it speeds along the production machines. Instead, direct human observation must be performed on the end product, resulting in a production control solution that is slow, inefficient, and simply unable to respond to constantly changing process variables in a timely manner. What is needed is an automated on-line system for measuring, analyzing, and valuating visual characteristics of paper in a manner that closely approximates and complements human assessments. The system should be able to provide a single and consistent valuation, or index of the visual properties of the web.
Attempts have been made to provide an on-line system for measuring formation and other visual characteristics of paper. Formation is most commonly measured on-line with single point light or laser transmission. However, this technique does not allow proper correlation with human visual assessment or with off-line formation measurement testers, which extract formation measurements from two-dimensional images. Also, the results of the off-line testers are only available after the paper has been made and sampled from reels, and, therefore come too late for any kind of on-machine control of paper formation.
Other attempts in the art have involved the use of backlighting the paper web as it speeds along the machine. In a paper entitled "A Device for Measuring the Orientation of Paper Formation", Foyn et al. discuss analyzing the orientation of formation across a paper web at web speeds up to about 4900 feet per minute. The paper discusses use of a stroboscope to illuminate the web while a CCD (Charge Coupled Device) camera with a 1/10,000 sec. shutter speed obtains data from the illuminated web. Data analysis is frequency-based and one-dimensional, and therefore lacks the ability to emulate human visual observation. Such systems suffer from various other drawbacks which limit their usefulness, particularly in high speed applications including, but not limited to, focus and motion jitter problems.
U.S. Pat. No. 5,068,799 to Jarrett, Jr. describes a method for detecting flaws in a continuous moving web of cloth material moving at speeds of up to 600 feet per minute by using a high intensity light to backlight the material. Data obtained with a video camera is then compared against stored templates of known flaws. However, the system is not useful in high speed on-line applications for providing real time control capability.
It is therefore an object of the present invention to provide a method and apparatus for measuring visual characteristics of a moving web of paper, including a web forming on a paper machine wire, and analyzing the resulting data on-line and during production to enable machine operators to make necessary control adjustments in a consistent and efficient manner.
Another object of the present invention is to provide a method and apparatus for on-line measurement of visual characteristics of a moving web of paper, including a forming web on a paper machine wire, and on-line analysis of the resulting data in a manner that closely approximates human assessment of the web's visual characteristics.
Another object of the present invention is to provide a method and apparatus capable of measuring visual characteristics of a paper web and producing an objective and consistent valuation, or index, of the web's visual characteristics.
Another object of the present invention is to provide a method and apparatus for measuring visual characteristics of a paper web on-line while the web is moving at speeds up to 5,000 feet per second.
Another object of the present invention is to provide a method and apparatus for measuring, analyzing, and displaying visual characteristics of a high speed web of paper with a video camera that does not induce motion jitter in the captured image.
Another object of the present invention is to provide a method and apparatus for measuring visual characteristics of a high speed web of paper with a video camera, and keeping the web in focus with the camera even as the web flutters.
Another object of the present invention is to provide a method and apparatus for on-line determination of a mathematical formation index corresponding to formation consistency in a moving web of paper.
Finally, it is an object of the present invention is to provide a method and apparatus for on-line determination of the areal composition of flocs and voids within a web of paper moving at a high speed on a papermaking machine.